The present disclosure relates to methods of treating electrodes, particularly electrodes for gas sensors.
Many sensors, such as those used for measuring nitrogen oxides (NOx) in exhaust gases, employ electrochemical methods. With the electrochemical sensing method, there are two basic principles involved in gas sensing: the polarographic principle and the Nernst principle. Typically, an exhaust gas sensor utilizing an electrochemical method comprises an electrochemical pump cell and an electrochemical motive force cell (“emf”) in operable communication.
The pump cell operates according to the polarographic principle. With the polarographic principle, the sensors utilize electrolysis whereby ions are sensed through a diffusion limiting current for electrolyte systems. Generally, a sensor employing the polarographic principle is composed of a pair of current pumping electrodes where both electrodes are in contact with an oxide conductive solid electrolyte and one electrode is in contact with a gas diffusion limiting means. The gas diffusion limiting means in conjunction with the pump electrodes create a limiting current which is linearly proportional to the measured gas concentration in the sample.
The emf cell operates with the Nernst principle, which describes the conversion of chemical into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (“sensing electrode”), and a porous electrode exposed to the partial pressure of a known gas (“reference gas electrode”). Sensors used in automotive applications often employ a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amount of a particular gas, such as NOx for example, that is present in an automobile engine's exhaust. Also, such a sensor may have a ceramic heater to help maintain the sensor's ionic conductivity. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia electrolyte, according to the Nernst equation:
  E  =            (                        -          RT                          4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
pO2ref=oxygen partial pressure of the reference gas
pO2=oxygen partial pressure of the exhaust gas
By combining the cell using a polarographic method (“pump cell”) and the cell using emf (“emf cell”) into one sensor, the sensor can be manufactured economically. A known type of exhaust sensor that can contain both a pump cell and an emf cell is a planar sensor employing a planar sensing element. The planar sensor is formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner.
In operation, the pump cell reduces oxygen gas to oxygen ions while allowing the gas to be measured (e.g. NOx) to pass to the emf cell. One problem with previous pump electrode designs is that the metals at the electrode surfaces (e.g., platinum) can interact with the gas to be measured (e.g. NOx), possibly giving misleading readings. There thus remains a need for additional methods for modifying the surface of an electrode and for gas sensors comprising such surface-modified electrodes.